The invention relates generally to seismic prospecting and, more specifically, to telemetry between a master electronics module, such as a data collector, and one or more slave electronics modules, such as seismic sensor data acquisition modules.
Conventional geophones and hydrophones used in seismic prospecting each have a dedicated two-wire connection to conduct analog seismic signals to acquisition/conversion circuitry. The analog signals from one or more remote seismic sensors (hydrophones, geophones, or other seismic sensors) are sampled and converted to a series of digital values by the acquisition/conversion circuitry. The acquisition/conversion circuitry is typically configurable to, for example, adjust the sampling rate, alter any digital filtering or other digital signal processing parameters, or perform diagnostics.
One or more of these acquisition/conversion circuits are connected to a data collection unit. Each data collection unit collects the series of digital values for all the seismic sensors connected to all the acquisition/conversion units connected to it. The data collection unit passes that data to a seismic recording system, including a system controller, over a high-speed data link, such as a fiber-optic cable.
Conventionally, however, the digital interface between an acquisition/conversion unit and a data collection unit has comprised at least four wires in two pairs: two wires (one pair) used for a digital command signal to the acquisition/conversion unit and two wires (the other pair) for the digital seismic data from the acquisition/conversion unit. In addition, power is supplied to the acquisition/conversion unit over the xe2x80x9cphantom pairxe2x80x9d formed by the two pairs of telemetry wires or over separate dedicated power wires.
Although the conventional four-wire telemetry works, it does have shortcomings. First, the weight of a cable depends on the number of wires and the concomitant amount of copper it contains. Second, the diameter of a cable depends on the number and size of wires it encases. Third, more wires require more connections to be made, which increases the chances of incorrect or unreliable connections.
It should be clear that there is a need for a smaller, lightweight, standard physical interface that can send commands and deliver power from a data collector to one or more remote sensor acquisition conversion units and transmit seismic data from the sensor acquisition/conversion units to a data collector.
The shortcomings of conventional seismic telemetry systems are overcome and the needs satisfied by a two-conductor bidirectional digital seismic interface having features of the invention. The interface comprises a two-conductor line connected between a master electronics module, such as a data collection module, and a slave electronics module, such as a seismic sensor electronics module. Digital commands are transmitted from the master electronics module to the slave sensor electronics module in one direction along the two-conductor line. Digital data from the slave sensor module are transmitted back to the master module in the opposite direction over the same pair of conductors to form a bidirectional interface. The slave module includes a phase-locked loop that derives clock information from the outbound command signal to keep the loop locked for coherent data acquisition and to derive a synchronization, or sync, point for properly decoding commands issued by the master. Block codes used by the master to encode the command bits that constitute a given command are selected to guarantee a level transition coincident with the sync point in the slave and to minimize dc drift. In this way, the slave remains synchronized with the master to enable synchronized bidirectional telemetry.
In a preferred version of the telemetry interface, the master transmits a synchronization pattern to lock the phase-locked loops in the slaves and to establish the sync point. Portions of outbound command bits and inbound data are confined to individual fixed-duration frames. Complete commands and data are apportioned among consecutive frames.
The slaves are preferably powered by a dc power supply at the master connected across the two-conductor pair. In this way, only two conductors are needed to handle bidirectional telemetry and to supply power, instead of the conventional four or more. This allows digital sensors to be used in place of analog sensors with only minor modifications of master and slave electronics.